Solebury Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Solebury Mountain Observatory clear sky chart from 2005-11-06 to 2025-08-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 7% 2 6% 4 4 4 3 5% 7% 40%	125831
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	122232
Transparency	Day	42% 5% 12% 22% 17% 3	129359
	Night	41% 5% 11% 23% 17% 3	126791
Seeing	Day	23% 14% 29% 21% 10% 3	15923
	Night	20% 12% 31% 27% 9% 1	25127

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 7% 2 5% 3 3 3 3 4 5% 50%	3597
	11th to 20th	Day	14% 9% 3 9% 4 4 3 3 5% 5% 40%	3598
	21st to end	Day	18% 9% 3 7% 4 4 3 3 4 5% 41%	4466
February	1st to 10th	Day	17% 8% 2 7% 4 3 5% 3 6% 7% 38%	1800
	11th to 20th	Day	16% 9% 3 7% 6% 3 4 3 4 6% 39%	1912
	21st to end	Day	19% 9% 3 6% 4 3 4 3 4 5% 39%	1650
March	1st to 10th	Day	22% 5% 2 4 4 4 3 2 4 5% 44%	4476
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 3 5% 6% 45%	4931
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 43%	5825
April	1st to 10th	Day	18% 7% 3 7% 4 3 4 3 5% 5% 40%	2400
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 5% 7% 41%	2374
	21st to end	Day	16% 6% 2 7% 4 5% 4 3 6% 6% 41%	2400
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 4 5% 8% 30%	5866
	11th to 20th	Day	25% 8% 3 6% 4 4 4 3 5% 6% 33%	6318
	21st to end	Day	21% 6% 2 5% 4 4 4 4 6% 8% 36%	6949
June	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 37%	2800
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 10% 38%	6360
	11th to 20th	Day	12% 7% 3 7% 5% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	12% 7% 3 6% 4 4 5% 4 6% 9% 40%	6891
August	1st to 10th	Day	21% 5% 2 6% 4 5% 5% 4 6% 7% 34%	4139
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 3 5% 9% 43%	3730
	21st to end	Day	12% 5% 2 5% 3 4 4 3 5% 7% 49%	4180
September	1st to 10th	Day	15% 6% 2 5% 5% 4 3 3 6% 7% 44%	2280
	11th to 20th	Day	13% 4 2 4 4 3 3 3 6% 8% 50%	2237
	21st to end	Day	19% 7% 2 6% 5% 5% 4 3 4 6% 40%	2090
October	1st to 10th	Day	19% 6% 2 4 4 3 4 3 4 7% 43%	1920
	11th to 20th	Day	14% 5% 2 5% 3 4 4 3 4 7% 48%	1900
	21st to end	Day	19% 5% 3 5% 2 3 4 3 5% 6% 45%	2000
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 3 6% 53%	1761
	11th to 20th	Day	17% 5% 2 5% 3 3 3 4 5% 5% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 4 3 3 5% 8% 45%	1675
December	1st to 10th	Day	15% 8% 2 5% 4 4 4 4 5% 7% 43%	2600
	11th to 20th	Day	22% 9% 1 4 4 3 3 2 4 5% 43%	2986
	21st to end	Day	17% 5% 3 6% 5% 5% 5% 4 5% 6% 39%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 50%	6349
	11th to 20th	Night	14% 8% 2 6% 4 4 3 3 4 5% 48%	6272
	21st to end	Night	18% 9% 2 6% 3 3 3 3 4 5% 43%	6490
February	1st to 10th	Night	17% 9% 3 5% 4 4 3 4 5% 6% 41%	2600
	11th to 20th	Night	18% 8% 3 6% 4 4 3 4 4 5% 41%	2373
	21st to end	Night	18% 8% 3 5% 3 3 4 4 4 6% 42%	1980
March	1st to 10th	Night	20% 7% 2 5% 3 3 3 2 5% 5% 44%	5364
	11th to 20th	Night	13% 8% 3 6% 3 3 3 3 4 5% 50%	5025
	21st to end	Night	17% 7% 2 7% 4 4 4 3 5% 5% 43%	5105
April	1st to 10th	Night	18% 7% 3 6% 4 4 4 4 5% 6% 39%	2000
	11th to 20th	Night	17% 7% 2 6% 5% 3 4 4 5% 6% 41%	1975
	21st to end	Night	18% 7% 1 4 3 3 3 3 6% 7% 45%	2000
May	1st to 10th	Night	25% 9% 3 6% 3 4 4 4 6% 8% 29%	4117
	11th to 20th	Night	24% 9% 3 7% 4 3 3 3 5% 6% 34%	3575
	21st to end	Night	22% 6% 2 6% 4 3 4 3 6% 8% 37%	3926
June	1st to 10th	Night	17% 6% 2 6% 4 4 4 4 5% 7% 42%	1600
	11th to 20th	Night	14% 5% 2 6% 5% 6% 4 4 6% 9% 40%	1600
	21st to end	Night	12% 4 2 5% 5% 4 5% 4 7% 9% 44%	1600
July	1st to 10th	Night	10% 5% 3 8% 6% 6% 5% 5% 6% 10% 37%	3600
	11th to 20th	Night	15% 8% 3 9% 5% 5% 4 5% 7% 9% 32%	3600
	21st to end	Night	12% 5% 2 5% 4 5% 4 4 6% 8% 44%	3957
August	1st to 10th	Night	23% 6% 3 6% 4 4 5% 3 6% 8% 33%	2861
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 6% 7% 40%	3083
	21st to end	Night	11% 5% 2 5% 3 4 4 3 6% 7% 50%	3476
September	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 5% 47%	1900
	11th to 20th	Night	12% 6% 2 4 4 3 3 2 4 6% 53%	1942
	21st to end	Night	18% 7% 2 5% 4 4 4 4 4 7% 41%	2090
October	1st to 10th	Night	19% 6% 3 5% 4 3 4 3 4 5% 44%	2256
	11th to 20th	Night	15% 4 2 4 3 3 3 2 5% 6% 51%	2280
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 6% 46%	2595
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 54%	2537
	11th to 20th	Night	18% 7% 2 4 4 3 3 3 4 4 49%	2600
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 44%	2720
December	1st to 10th	Night	13% 8% 3 5% 3 4 3 3 5% 6% 47%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	21% 7% 3 6% 4 3 4 3 4 5% 41%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	35% 3 9% 28% 22% 2	3598
	11th to 20th	Day	44% 5% 9% 21% 15% 6%	3628
	21st to end	Day	47% 4 10% 22% 16% 1	4466
February	1st to 10th	Day	44% 4 14% 24% 12% 2	1800
	11th to 20th	Day	47% 3 9% 23% 16% 3	1921
	21st to end	Day	47% 5% 10% 22% 13% 3	1650
March	1st to 10th	Day	43% 3 9% 23% 15% 7%	4479
	11th to 20th	Day	39% 4 12% 28% 13% 5%	4931
	21st to end	Day	43% 4 10% 15% 23% 6%	5825
April	1st to 10th	Day	45% 5% 10% 20% 15% 5%	2400
	11th to 20th	Day	41% 6% 12% 21% 17% 3	2374
	21st to end	Day	42% 5% 10% 20% 18% 4	2400
May	1st to 10th	Day	53% 5% 14% 17% 10% 1	5877
	11th to 20th	Day	52% 5% 9% 21% 12% 1	6322
	21st to end	Day	46% 6% 10% 23% 15% 1	6949
June	1st to 10th	Day	40% 8% 14% 22% 15% 1	2800
	11th to 20th	Day	43% 8% 14% 22% 11% 2	2800
	21st to end	Day	39% 8% 14% 23% 16% 1	2800
July	1st to 10th	Day	42% 7% 14% 20% 15% 2	6290
	11th to 20th	Day	45% 6% 18% 20% 10% 1	6285
	21st to end	Day	40% 5% 11% 20% 23% 1	6693
August	1st to 10th	Day	48% 7% 13% 21% 11% 1	4126
	11th to 20th	Day	38% 6% 13% 28% 14% 1	3766
	21st to end	Day	34% 5% 10% 27% 22% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 21% 3	2280
	11th to 20th	Day	33% 5% 11% 24% 24% 3	2237
	21st to end	Day	46% 5% 9% 17% 20% 3	2090
October	1st to 10th	Day	38% 4 9% 18% 25% 5%	3193
	11th to 20th	Day	25% 4 11% 25% 30% 6%	3140
	21st to end	Day	26% 3 14% 24% 28% 5%	3277
November	1st to 10th	Day	34% 4 13% 25% 22% 3	1761
	11th to 20th	Day	38% 4 10% 24% 21% 3	1800
	21st to end	Day	38% 3 14% 30% 12% 3	1675
December	1st to 10th	Day	40% 4 15% 27% 12% 2	2600
	11th to 20th	Day	45% 3 11% 27% 11% 2	2986
	21st to end	Day	43% 5% 11% 22% 16% 2	3960
January	1st to 10th	Night	35% 4 8% 26% 21% 5%	6349
	11th to 20th	Night	39% 3 13% 23% 15% 7%	6317
	21st to end	Night	44% 3 10% 22% 18% 4	6490
February	1st to 10th	Night	44% 5% 13% 25% 11% 2	2600
	11th to 20th	Night	45% 5% 9% 20% 18% 4	2375
	21st to end	Night	43% 4 8% 21% 19% 4	1980
March	1st to 10th	Night	42% 4 9% 23% 12% 9%	5374
	11th to 20th	Night	38% 4 14% 25% 15% 4	5026
	21st to end	Night	45% 4 7% 16% 23% 6%	5105
April	1st to 10th	Night	44% 6% 11% 19% 16% 4	2000
	11th to 20th	Night	43% 6% 9% 19% 19% 3	1975
	21st to end	Night	39% 5% 10% 21% 20% 5%	2000
May	1st to 10th	Night	54% 5% 12% 17% 10% 2	4123
	11th to 20th	Night	52% 6% 12% 16% 13% 2	3576
	21st to end	Night	45% 6% 12% 22% 12% 2	3927
June	1st to 10th	Night	40% 6% 12% 21% 20% 1	1600
	11th to 20th	Night	40% 7% 14% 25% 12% 1	1600
	21st to end	Night	35% 8% 15% 25% 14% 2	1600
July	1st to 10th	Night	43% 8% 16% 18% 15% 1	3560
	11th to 20th	Night	48% 6% 15% 20% 10% 1	3552
	21st to end	Night	38% 5% 14% 23% 20% 1	3840
August	1st to 10th	Night	50% 6% 18% 16% 8% 1	2850
	11th to 20th	Night	42% 6% 12% 26% 13% 1	3113
	21st to end	Night	33% 5% 10% 29% 21% 1	3476
September	1st to 10th	Night	39% 7% 11% 20% 21% 2	1900
	11th to 20th	Night	33% 5% 11% 22% 26% 4	1942
	21st to end	Night	42% 5% 8% 21% 21% 2	2090
October	1st to 10th	Night	41% 4 10% 22% 21% 2	3763
	11th to 20th	Night	25% 3 9% 29% 29% 4	3779
	21st to end	Night	29% 3 11% 31% 23% 3	4268
November	1st to 10th	Night	33% 4 12% 24% 23% 4	2537
	11th to 20th	Night	39% 4 9% 23% 20% 5%	2600
	21st to end	Night	39% 4 13% 26% 16% 2	2720
December	1st to 10th	Night	40% 3 12% 31% 13% 2	4580
	11th to 20th	Night	45% 4 10% 24% 13% 3	5208
	21st to end	Night	46% 4 10% 21% 17% 3	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 24% 28% 14% 2 1	200
	11th to 20th	Day	37% 24% 30% 8% 1	207
	21st to end	Day	25% 26% 32% 12% 4	330
February	1st to 10th	Day	31% 25% 32% 8% 3	300
	11th to 20th	Day	33% 22% 33% 10% 2	460
	21st to end	Day	32% 21% 30% 13% 3	414
March	1st to 10th	Day	31% 20% 30% 16% 3	500
	11th to 20th	Day	27% 24% 31% 14% 3 1	500
	21st to end	Day	28% 24% 29% 13% 5% 1	550
April	1st to 10th	Day	31% 21% 30% 12% 5% 1	500
	11th to 20th	Day	26% 21% 31% 13% 8% 2	492
	21st to end	Day	28% 18% 30% 17% 4 2	500
May	1st to 10th	Day	31% 19% 28% 15% 6% 2	503
	11th to 20th	Day	25% 14% 32% 20% 9% 1	527
	21st to end	Day	21% 9% 27% 24% 16% 3	573
June	1st to 10th	Day	19% 11% 30% 24% 14% 3	530
	11th to 20th	Day	21% 11% 28% 26% 12% 3	530
	21st to end	Day	17% 10% 31% 27% 13% 3	530
July	1st to 10th	Day	13% 4 24% 32% 21% 6%	525
	11th to 20th	Day	9% 4 26% 32% 23% 6%	524
	21st to end	Day	14% 7% 31% 31% 15% 3	576
August	1st to 10th	Day	12% 4 26% 34% 17% 6%	502
	11th to 20th	Day	14% 5% 22% 33% 22% 5%	479
	21st to end	Day	14% 6% 27% 29% 15% 9%	539
September	1st to 10th	Day	16% 9% 24% 26% 16% 8%	490
	11th to 20th	Day	14% 7% 26% 28% 16% 9%	490
	21st to end	Day	24% 12% 25% 23% 11% 5%	490
October	1st to 10th	Day	23% 8% 29% 19% 13% 7%	490
	11th to 20th	Day	18% 14% 30% 25% 10% 2	490
	21st to end	Day	23% 15% 30% 18% 12% 2	492
November	1st to 10th	Day	20% 11% 32% 24% 11% 3	395
	11th to 20th	Day	24% 16% 34% 20% 5% 1	400
	21st to end	Day	24% 17% 38% 14% 6%	275
December	1st to 10th	Day	30% 23% 31% 10% 7%	200
	11th to 20th	Day	40% 19% 25% 11% 6%	200
	21st to end	Day	30% 17% 29% 15% 8%	220
January	1st to 10th	Night	22% 19% 37% 19% 3	800
	11th to 20th	Night	24% 16% 35% 22% 3	800
	21st to end	Night	20% 19% 39% 20% 1	880
February	1st to 10th	Night	23% 16% 35% 24% 2	800
	11th to 20th	Night	27% 19% 37% 15% 1	783
	21st to end	Night	25% 18% 37% 18% 2	660
March	1st to 10th	Night	24% 21% 32% 21% 3	800
	11th to 20th	Night	24% 18% 36% 19% 4	800
	21st to end	Night	21% 16% 40% 18% 5%	880
April	1st to 10th	Night	24% 18% 37% 17% 4	800
	11th to 20th	Night	21% 14% 38% 22% 5%	791
	21st to end	Night	21% 13% 34% 26% 5%	800
May	1st to 10th	Night	26% 14% 30% 24% 6%	637
	11th to 20th	Night	22% 9% 36% 26% 6%	394
	21st to end	Night	17% 7% 25% 37% 12% 1	433
June	1st to 10th	Night	17% 6% 29% 36% 12% 1	400
	11th to 20th	Night	14% 4 31% 40% 12%	400
	21st to end	Night	12% 3 26% 47% 12% 1	400
July	1st to 10th	Night	11% 6% 22% 40% 19% 2	390
	11th to 20th	Night	13% 3 22% 43% 19% 1	388
	21st to end	Night	10% 4 25% 43% 18%	444
August	1st to 10th	Night	14% 2 16% 45% 21% 2	579
	11th to 20th	Night	13% 2 21% 37% 25% 2	736
	21st to end	Night	15% 4 17% 41% 21% 2	836
September	1st to 10th	Night	17% 3 25% 35% 18% 3	760
	11th to 20th	Night	14% 4 22% 35% 21% 4	760
	21st to end	Night	23% 5% 23% 31% 17% 1	760
October	1st to 10th	Night	22% 8% 25% 26% 16% 3	760
	11th to 20th	Night	16% 10% 29% 32% 12%	760
	21st to end	Night	21% 11% 33% 25% 9%	836
November	1st to 10th	Night	15% 10% 34% 30% 11% 1	780
	11th to 20th	Night	22% 15% 30% 26% 7%	800
	21st to end	Night	20% 15% 34% 24% 7%	800
December	1st to 10th	Night	25% 16% 31% 21% 5% 1	800
	11th to 20th	Night	23% 14% 35% 24% 5%	800
	21st to end	Night	23% 15% 32% 27% 3	880

Page generated at 2025-08-02 01:32:28. Version ac07

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